bgpd: transfer packets from peer stub to actual peer

bgpd: transfer packets from peer stub to actual peer

During transition from OpenConfirm -> Established, we wipe the peer stub's
output buffer. Because thread.c prioritizes I/O operations over regular
background threads and events, in a single threaded environment this ordering
meant that the output buffer would be happily empty at wipe time.  In MT-land,
this convenient coincidence is no longer true; thus we need to make sure that
any packets remaining on the peer stub get transferred over to the peer proper.

Also removes misleading comment indicating that bgp_establish() sends a
keepalive packet. It does not.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>